A. Field of the Invention
This invention relates to integrated circuits, and more particularly to integrated circuit (I.C.) integrators employing active elements which can be tuned to achieve a desired frequency response.
B. Description of the Prior Art
Historically, filtering has been accomplished by using inductors and capacitors to form passive LC circuits. Such circuits utilize the energy storage capabilities of both the inductors and capacitors, and have been extensively used.
More recently, integrated circuits and resistor-capacitor (RC) active filters have emerged, both having distinct advantages over the older discrete element filters. Whereas LC filters employ large and bulky inductors and attenuated input signals, RC active filters do not need the large inductor elements, and can provide gain to prevent a loss in signal amplitude.
The advent of active RC filters has also been accompanied by some practical problems. For example, both passive LC and active RC filters depend upon the absolute values of their respective components to achieve accurate filtering. In both cases, in order to obtain the desired frequency selective characteristics, either expensive high-precision components have to be selected, or the components have to be trimmed, a slow and expensive manufacturing operation. Furthermore, while capacitors are susceptible to integrated circuit techniques whereas inductors are not, the tolerances inherent in the manufacturing processes for integrated circuits are such that it is difficult to form either resistors or capacitors with sufficient accuracy of absolute values to satisfy the requirements of active RC circuits.
The surface area occupied by I.C. capacitors imposes an additional limitation on their use in active filter circuits. The manufacturing yield of an I.C. circuit decreases as the area occupied by the circuit increases, as a result of which I.C. design is oriented toward minimizing area. For high frequency filtering, the time constants of the required integrators are small enough that they can be made in I.C. form with acceptable yields. However, at low frequencies in the audio range, the time constants are so large that external capacitors have been used instead of I.C. capacitors. I.C. capacitors have generally been restricted to values below a few thousand picofarads because of the large area occupied by larger capacitors.
There thus exists a need for an approach that can achieve the benefits of I.C. technology for an integrating function in the low frequency range, but which avoids the uneconomical loss in yield accompanying the use of large capacitors. One such technique is described in a paper by Khen-Sang Tan and Paul R. Gray, "High-Order Monolithic Analog Filters Using Bipolar JFET Technology", Digest of Technical Papers, 1978 IEEE International Solid-State Circuits Conference, pages 80-81, 268. This paper describes filters with a plurality of similary constructed I.C. integrator circuits. Each integrator circuit has an input stage with a transconductance defined by a network of high pinch-off junction field effect transistors (JFETS), and an output gain stage with a feedback capacitor. Multiplying the input stage resistance by the value of the feedback capacitor gives an equivalent time constant for each integrator. By varying the biasing current for the JFET input stage, the transconductance of that stage, and hence the gain constant of the entire integrator, can be varied. Biasing current sources for the various integrators are matched, and their respective capacitors are ratioed. By connecting the integrator circuits in a phase-lock-loop system with a voltage-controlled-oscillator (VCO) using integrators whose characteristics were precisely matched with the filter integrators, and having an oscillation frequency directly related to the desired filter frequency response, the cutoff frequency of the resulting filter was reported to track the external frequency, independent of process and temperature variations affecting the absolute magnitudes of the individual integrator gain constants.
The above circuit represents a significant improvement, in that it avoids the use of a single large capacitor by employing a plurality of integrators having low transconductance input stages, to achieve a large time constant. This technique is better suited for I.C. fabrication than is the former practice. However, it has been found to have certain deficiencies. Because it employs ion-implanted JFETs which exhibit large processing variations, there is a large variation in the absolute values of the integration constants for the various integrator circuits. Furthermore, since it utilizes the weak transconductance dependence of JFETs on drain current in their linear (non-saturated) range of operation to vary the value of integration constants, the tunable range of the resulting filter is quite small. Even large changes in drain current will not change the transconductance very much. Also, since JFETs are basically non-linear devices, the output current of the input transconductance stage is not linear with respect to the input voltage.
There is thus still a need for an economical I.C. integrator which can operate at low frequencies, and has a predictable and controllable frequency response despite considerable processing and temperature variations.